Former FDA Commissioner Scott Gottlieb told CNBC he isn’t convinced that vaping nicotine is carcinogenic, calling out a study that pointed toward the opposite conclusion.

“I’m skeptical that nicotine causes cancer,” Gottlieb, a former physician, said Monday on “Squawk Box.” “It might be a tumor promoter, [researchers] have said that there’s a potential that nicotine is a tumor promoter, but it doesn’t cause cancer.”

Gottlieb was reacting to a study released last Monday that said e-cigarette vapor with nicotine causes lung cancer and potentially bladder cancer in mice, damaging their DNA and leading researchers at New York University to conclude that vaping is likely “very harmful” to humans as well.

Out of 40 mice exposed in the study to e-cigarette vapor with nicotine over 54 weeks, 22.5% developed lung cancer and 57.5% developed precancerous lesions on the bladder. None of the 20 mice exposed to e-cigarette vaper without nicotine developed cancer over the four years they studied the mice, researchers said.

Gottlieb expressed doubt, saying the study was conducted on a small scale and isolated the effects of nicotine. The NYU researchers were not immediately available to comment on Gottlieb’s remarks.

However, Gottlieb still said he would expect inhaling vapor to cause damage to the lungs.

“That said, we had a signal when I was at the FDA of damage that vaping was causing to the lungs in an animal study” said Gottlieb, a Pfizer board member who is also a CNBC contributor. The results should be out soon on the FDA study, he said. “You can’t inhale something into the lungs that way on a repeated basis and not cause some damage to the lung.”

Gottlieb left the FDA in April.

The research into vaping comes at a time of increased scrutiny of e-cigarettes as underage use rises and U.S. health officials trace an outbreak of a deadly lung disease linked back to vaping, mostly THC, the active compound in marijuana. According to the CDC, some of the roughly 1,300 victims who have fallen ill, including at least 26 who have died, reported using only nicotine, leading doctors to say they can’t rule anything out.

E-cigarette makers marketed themselves as a safer alternative to smoking regular, combustible cigarettes that can help adult smokers kick the habit.